I rode in a bronco at the new york international auto show. It seemed to flex pretty well, especially once they hit the swaybar disconnect. Admittedly didn't have a 4Runner to compare against.
It's not so much the range of travel but the load distribution while articulating. If you put the springs all the way out at the ends of the axle beam, you don't take advantage of the potential of the beam to use the lever arm to help distribute the weight side to side.
This is a copy and paste from another discussion, but it explains the issue fairly well I think:
In a simple scenario where you have a 72" track width and springs with a constant spring rate mounted inboard at 48" width (1 foot in from each WMS), and a spring rate of 200lb/in and the axle is bearing the weight of 2500lb. On a level surface each tire is loaded with 1,250lb of weight.
Now let's assume an obstacle where frame remains level and one tire is on top of a 12" rock. So the axle system will be put at a 9.56* angle.
With a solid axle modeled as a continuous beam with two point loads (springs) and supported ends (tires) the force on the tire displaced upward will be
1783lb and the force on the down side will be
717 lbs. (The springs will be displaced 4" up on one side and 4" down on the other, but that does not calculate the ground force distribution which will be a more complex, but not terribly complex, beam with asymmetrical point loads).
With coilovers like the Bronco uses if we took an extreme assumption to better tease out the issue - let's assume that each coilover was attached to the very end point of the axle beam. The axle would not transfer any of the force from one side to the other. The force on the upper tire will be
2450lb and the lower tire will be
50lb.
In both of those examples the tires always stay on the ground. They flex the same amount. Visually it would look pretty similar. But in terms of load distribution - the inboard spring design performs much better. And that translates to significantly less imbalance on the vehicle body, less body motion as you travel over obstacles (less head toss and more stability), and ideally better overall confidence and traction as the tires remain more balanced.
It's a balancing act though because as you move the springs inward you also reduce the inherent stability to resist body roll in corners or on sidehills. The solution for that is KDSS or similar anti-roll mechanisms. That's why the Toyota KDSS system paired with the inboard coils was a really great overall performer. Especially as implemented in the LC300 and should be in the GX550.